Properties of localized polar cap flow enhancements and their connection to nightside poleward boundary intensifications and substorms

Tuesday, 16 December 2014
Ying Zou1, Yukitoshi Nishimura2, Larry R Lyons2, Kazuo Shiokawa3, Eric Donovan4, J. Michael Ruohoniemi5, Kathryn A McWilliams6 and Nozomu Nishitani7, (1)UCLA, Los Angeles, CA, United States, (2)University of California Los Angeles, Los Angeles, CA, United States, (3)Nagoya University, Solar terrestrial Environment Laboratory, Nagoya, Japan, (4)University of Calgary, Calgary, AB, Canada, (5)Virginia Tech, Blacksburg, VA, United States, (6)University of Saskatchewan, Saskatoon, SK, Canada, (7)Nagoya University, Nagoya, Japan
Previous radar observations have shown that polar cap flows are highly structured and that localized flow enhancements are related to nightside auroral disturbances. However, such studies are limited to available echo regions. Utilizing wide spatial coverage by an all-sky imager at Resolute Bay and simultaneous SuperDARN radar measurements, we determined properties of such localized flow enhancements and their statistical association with nightside auroral activity. We found that narrow flow enhancements are well collocated with airglow patches with substantially larger velocities (>~200 m/s up to ~700 m/s) than the weak large-scale polar cap convection. The flow widths are similar to the patch widths. During the evolution across nightside the polar cap, the flow directions and speeds are consistent with the patch propagation directions and speeds. These correspondences indicate that patches can visualize localized flow enhancements reflecting the flow width, speed and direction. Such associations were prevalent (~67%) in our survey and tended to be observed under By-dominated IMF. The typical flow channel speed, propagation time, and width within our observation areas were 600 m/s, tens of minutes, and 200-300 km, respectively. Localized flow enhancements were usually observed as a channel elongated in the noon-midnight meridian and directed towards pre-midnight (post-midnight) for +By (-By), and accounted for ~10-40% of the plasma transport across the entire polar cap. Utilizing airglow patches as tracers of localized flow enhancements, we also found that as localized polar cap structures move across the polar cap and impinge on the poleward boundary of the nightside auroral oval, they are followed by poleward boundary intensifications (PBIs), which can extend equatorward as auroral streamers, some of which further propagate equatorward and are followed by a substorm auroral onset. This suggests that localized flow enhancements within the polar cap can contribute to enhanced distant magnetotail reconnection, earthward flow channels in the plasma sheet, and substorm disturbances.